Field of the Invention
The present invention relates to a sewing machine such as a multi-thread chain stitch sewing machine, covering sewing machine, safety stitch sewing machine, or the like to perform sewing by means of a needle that moves up and down while sticking into a cloth on a needle plate and a looper that projects and withdraws laterally under the needle plate.
The multi-thread chain stitch sewing machine has one needle or a plurality of needles that hold a needle thread and move up and down while sticking into a cloth on a needle plate. Furthermore, the multi-thread chain stitch sewing machine has a looper that holds a looper thread and projects and withdraws laterally under a needle plate. This multi-thread chain stitch sewing machine operates as follows. First, when a needle thread is sent out by a fall of the needle and the movement of the needle is converted into a rise, the multi-thread chain stitch sewing machine operates so that the looper catches the loop of the needle thread left under the needle plate in synchronization with this rise.
Second, the multi-thread chain stitch sewing machine draws out the loop of the needle thread caught by the looper upon feeding a cloth back or forth on the needle plate. Third, when changing the looper so as to withdraw, the multi-thread chain stitch sewing machine catches the loop of the looper thread left by the needle that moves down in synchronization with the withdrawal. The multi-thread chain stitch sewing machine repeats these operations in order.
The covering stitch sewing machine performs such sewing operation in the same manner. The safety stitch sewing machine performs over-edge chain stitching on the edge of a cloth, and at the same time, performs the abovementioned multi-thread chain stitching along one side of the over-edge chain stitch.
FIG. 3 and FIG. 4 are plan views of the multi-thread chain stitches viewed from the back surface side of a cloth to be sewn. As shown in the figures, the looper thread 20 is caught in the loop of needle thread 10 that is exposed to the back surface of the cloth W at predetermined intervals corresponding to the amount of movement of the cloth W.
In the figures, the white arrows with black outlines show the direction of feeding of the cloth W. The needle thread 10 and looper thread 20 are intertwined with each other at a stitch by inserting the looper thread 20 into one loop (rear loop) of the needle thread 10 from the rear side of the direction of feeding. Then, the intertwined portion is passed through the front portion of the loop (front loop) adjacent to the rear loop and returned to the rear side. Thereafter, the intertwined portion is inserted into the rear loop again and turned back. Last, the intertwined portion is inserted into the front loop from the rear side of the direction of feeding. These processes are repeated, and then the intertwined portions form double rings formed of the looper thread 20 across the front loop and rear loop that are formed of needle thread 10 adjacent to each other in the direction of feeding.
The difference between the stitches shown in FIG. 3 and FIG. 4 arises due to the balance in tension between the needle thread 10 and looper thread 20. In the case where the tension of the needle thread 10 is increased to be greater than that of the looper thread 20, due to the tension of the needle thread 10 toward the surface side of the cloth W, the inserted portion of the looper thread 20 is strongly tightened. As a result, as shown in FIG. 3, a stitch in which the double rings stretch between the rear loop and front loop of the needle thread 10 is obtained.
To the contrary, in the case where the tension of the needle thread 10 is weakened to be less than that of the looper thread 20, the looper thread 20 is pulled against the relatively week tightening by the needle thread 10. As a result, as shown in FIG. 4, the double rings formed of the looper thread 20 stretch between the rear loop and front loop of the needle thread 10 and form a stitch in which the double rings overlap each other.
Such a form of stitches is selected depending on the kind of the cloth W to be sewn and sewing conditions such as a sewing portion. For example, to sew a thick cloth with high rigidity such as denim or the like, the stitch shown in FIG. 4 is selected in many cases in order to realize firm sewing without looseness. On the other hand, to sew a thin cloth with flexibility such as a cloth for a shirt, the stitch shown in FIG. 3 is selected in order to obtain a smooth texture.
However, to obtain the stitch shown in FIG. 3, as mentioned above, the tension of the needle thread 10 must be increased to be greater than that of the looper thread 20. If this is realized by increasing the tension of the needle thread 10, the cloth itself is tightened due to the high tension of the needle thread 10. Then, surface wrinkles called puckering occurs on the cloth after being sewn. Thereby, the finishing quality deteriorates.
To the contrary, in the case where the stitch is realized by reducing the tension of the looper thread 20, the looper thread sent out by the looper become loose. Then, it fails to catch the looper thread 20 when the needle moves down. Thereby, skip stitch occurs and the finishing quality deteriorates.
In order to solve the above problems and stably obtain the stitch shown in FIG. 3, a sewing machine provided with a retainer needle has been conventionally made practicable. This retainer needle is mounted together with a sewing needle to the needle bar so as to be positioned closer to the projection side of the looper than the sewing needle. The retainer needle reaches the position under the needle plate prior to the sewing needle when the needle bar moves down. On the other hand, the retainer needle comes out from the position under the needle plate after the sewing needle when the needle bar moves up.
According to this construction, the looper thread 20 sent out by the projection of the looper is caught by the retainer needle which reaches the position under the needle plate prior to the needle at the projection side of the looper ahead of the needle (in the case where a plurality of needles are provided, outside the outermost needle). The needle moves up in synchronization with the next projection of the looper, and this catch is maintained until the tightening by the needle thread 10 is started. This result is shown in FIG. 3. Between the front and rear loops of the needle thread 10 to be exposed to the back surface of the cloth W, the double rings of the looper thread 20 gradually spread. Then, a stitch is stably formed. It becomes unnecessary to adjust the tension of the needle thread 10 and looper thread 20. Furthermore, sewing failures such as the abovementioned puckering and skip stitch do not occur.
Thus, the sewing machine with a retainer needle has a simple structure in which the retainer needle is only attached to the needle bar for moving the needle up and down. In addition, the sewing machine with a retainer needle can stably perform multi-thread chain stitching (including safety stitching) or covering stitching regardless of the kind of the sewing cloth.
However, the retainer needle is constructed so as to reach the position under the needle plate by penetrating the cloth W together with the needle to catch the looper thread 20. When sewing by such a sewing machine with a retainer needle, as shown in FIG. 3, holes H, H . . . caused by penetration of the retainer are formed in line along one side of the multi-thread chain stitch (the projection side of the looper). Therefore, for uses requiring high finishing quality, the existence of these holes comes into question.
Recently, in order to make ironing after washing unnecessary, sewn products (such as shirts, handkerchiefs, and others) using shape-memory cloth have been made practicable. In the case where a sewing machine with a retainer needle is used for sewing each portion of such a type of cloth, the abovementioned holes H, H . . . are left as unnecessary traces keeping their form when they are formed. Therefore, an excellent finishing quality cannot be obtained.
The present invention is made in order to solve the above problems. The object of the invention is to provide a sewing machine which can form a multi-thread chain stitch or covering stitch in which the looper thread is intertwined well with the needle thread on the back surface of a cloth without undesirable traces. Also, the object of the invention is to provide a sewing machine that can stably obtain sewn products of a good quality.
The sewing machine according to the first aspect of the invention comprises one needle or a plurality of needles that hold a needle thread and move up and down at a predetermined stroke penetrating of a needle plate; a looper that holds a looper thread, projects and withdraws laterally under the needle plate, and catches the needle thread left at the down position of the needle when the looper projects; and a retainer that moves forward and rearward within a plane under and roughly in parallel with the needle plate, and when moving rearward, crosses the travel path of the looper and catches the looper thread held by the looper at the projection side of the looper ahead of the needle.
In the sewing machine according to the first aspect of the invention, the retainer, which moves forward and rearward within a plane roughly in parallel with the needle plate while interlocked with the rise and fall of the needle and the lateral movement of the looper, is disposed under the needle plate. That is, in the first aspect of the invention, a retainer is disposed which moves without interference with the movements of the needle and looper and without an influence on a cloth on the needle plate. This retainer catches the looper thread at the projection side of the looper ahead of the needle when moving rearward. The retainer keeps this catch until it withdraws forward. The retainer maintains the looper thread under tension, and tightens the needle thread. Thereby, the sewing machine of the invention stably obtains stitches in which the looper thread intertwines with the needle thread in a loose condition without traces left on the cloth.
In the sewing machine according to the second aspect of the invention, the retainer according to the first aspect of the invention comprises a retainer base that is disposed under the needle plate and swings backward and forward in accordance with power transmission from the sewing machine""s main shaft; and a retainer rod that is fixed at the base part on said retainer base and curves under the lower surface of the needle plate.
According to the second aspect of the invention, a retainer base is constructed so as to swing backward and forward in accordance with power transmission from the sewing machine""s main shaft. The retainer rod that curves at a proper radius of curvature is provided on the retainer base. The retainer rod is provided on the retainer base with its curved portion shaped along the lower surface of the needle plate. The retainer is comprised of this retainer base and retainer rod. In the second aspect of the invention, the retainer base is made to swing, and the retainer rod is made to project and withdraw to catch the looper thread and release the catch. Furthermore, in the second aspect of the invention, the entire retainer including the retainer base and the retainer rod is disposed within a limited space such as the inside of a cylindrical bed under the needle plate. Thereby, the second aspect of the invention can be applied to models in which the space under the needle plate is limited.
In the sewing machine according to the third aspect of the invention, the timing of operation of the retainer according to the first or second aspect of the invention is set so that the condition where the looper thread is caught due to the rearward movement is maintained at least until the tip end of the looper that projects in accordance with the rise of the needle crosses the travel path of the needle.
In the invention, by the operation of the retainer at the abovementioned timing of operation, the needle thread is securely tightened before tightening the looper thread. Thereby, the third aspect of the invention correctly realizes stitches in a condition where the looper thread is loose.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.